The promotion of physical activity (PA) is a priority in our society. Self-determination theory (SDT) has evolved considerably in recent years with the aim of improving the motivation of people and their PA levels. This thesis is composed of three separate studies. The first study is based on a validation of the questionnaire 7-Day PAR using accelerometer data and analyzing the relationship of these data with criterial variables such as physical fitness and percentage of body fat mass. The second study attempts to ascertain the adolescents’ PA levels, differentiating by gender and segments of the day . Finally, in the third study, the effects of a school-based-SDT intervention to improve motivation and patterns of PA are tested.To validate the 7 Day-PAR we counted with 123 students (73 girls and 50 boys) aged between 13 and 17 years (M = 14.85, SD = .87). In the study of the PA levels by gender the participants were 206 students (118 girls and 88 boys) between 14 and 17 years (M = 15.07, SD = .86). For the intervention study there were 88 students (52 girls and 36 boys), of which 59 belonged to the control group and 29 to the experimental group, aged between 14 and 17 years (M = 14.67, SD = .66). The interventions of the study were a teaching unit (TU) of physical education (PE), related to the block content of fitness and health, a program of extracurricular PA and meetings with parents. In all these interventions, specific strategies to promote motivation and PA based on SDT were carried out. The analyses used for validation of the 7 Day-PAR were bivariate correlation analysis and linear regression analysis. For study 2, MANCOVAs and ANCOVAs of continuation were calculated. In the latest study, different nonparametric tests as Mann-Whitney U-test, Friedman test and the Wilcoxon test were performed.The results of the 7 Day-PAR validation indicate greater validity of this instrument for the very hard PA, while showing some limitations for lower intensity physical activities. For study 2, the boys showed more vigorous PA, whereas the girls showed more sedentary activity during preschool segments (6: 30-8: 30 h) and evening (18: 31-21: 00 h) on weekdays. In addition, the boys also showed higher levels of vigorous PA in school segments (8: 31-14: 30 h), lunch (14: 31-16: 00 h) and afternoon on weekdays and weekends (16: 01-18: 30 h). The results of the intervention study showed statistically significant differences in favor of the experimental group in parents and peers autonomy support, integrated regulation in PE, autonomy, intrinsic and identified regulation in leisure-time PA, moderate PA, hard PA and very hard PA throughout the entire study. By contrast, the control group showed greater amotivation in PE andleisure-time PA. The motivational effects were maintained throughout the study but the effects on PA levels disappeared at 6 months following intervention.In view of the results it is necessary to unify the PA intensity levels of subjective and objective methods for the measurement of PA, pay attention to the contents of PE classes for a better promotion of PA, especially among girls, and introduce more long-lasting interventions to bring about behavior change within the school environment.